Mediator-containing mast cell granules have been shown to be ingested by connective tissue fibroblasts. This phagocytic process is both time and temperature dependent, and results in an increased rate of enzyme secretion from fibroblasts. The heparin matrix of the granule is degraded within the fibroblast following ingestion. This ability of fibroblasts to remove and degrade mast cell granules represents an important mechanism by which immediate hypersensitivity reactions are terminated. It also appears that mast cell products can alter fibroblast responses such as enzyme secretion and proteoglycan synthesis. In studies using human lymphocytes, mast cell granules were found to inhibit antigen- and lectin-induced blastogenesis. This effect persisted after histamine removal. The inhibitory factor was shown to be heparin proteoglycan. Thus, mast cell granules, by virtue of their heparin content, can inhibit lymphocyte function. Proteoglycans from three cloned granulated lymphocyte cell lines with natural killer activity and one cultured mast cell line were characterized. The proteoglycans from each cell line were distinct, with the granulated lymphocytes containing only chondroitin 4-sulfate and the mast line producing an oversulfated chondroitin 6-sulfate. These findings suggest that glycosaminoglycan profiles are useful biochemical markers in the characterization of diverse granulated cell lines. Cultured mast cells derived from mouse bone marrow produce an oversulfated chondroitin sulfate rather than heparin. This observation, along with the presence of characteristic surface markers and distinct responses to degranulating agents suggests that these cells are mucosal mast cells.